1. Field of the Invention
The present invention relates to acoustic sensors, including microphone arrays, and in particular, to amplifier circuits for differential microphone arrays.
2. Description of the Related Art
With the seemingly ever increasing popularity of cellular telephones, as well as personal digital assistances (PDAs) providing voice recording capability, it has become increasingly important to have noise canceling microphones capable of operating in noisy acoustic environments. Further, even in the absence of excessive background noise, noise canceling microphones are nonetheless highly desirable for certain applications, such as speech recognition devices and high fidelity microphones for studio and live performance uses.
Such microphones are often referred to as pressure gradient or first order differential (FOD) microphones, and have a diaphragm which vibrates in accordance with differences in sound pressure between its front and rear surfaces. This allows such a microphone to discriminate against airborne and solid-borne sounds based upon the direction from which such noise is received relative to a reference axis of the microphone. Additionally, such a microphone can distinguish between sound originating close to and more distant from the microphone.
For the aforementioned applications, so called close-talk microphones, i.e., microphones which are positioned as close to the mouth of the speaker as possible, are seeing increasing use. In particular, multiple microphones are increasingly configured in the form of a close-talking differential microphone array (CTDMA), which inherently provide low frequency far field noise attenuation. Accordingly, a CTDMA advantageously cancels far field noise, while effectively accentuating the voice of the close talker, thereby spatially enhancing speech quality while minimizing background noise. (Further discussion of these types of microphones can be found in U.S. Pat. Nos. 5,473,684, and 5,586,191, the disclosures of which are incorporated herein by reference.)
Optimum performance of a CTDMA system using multiple microphones is obtained when all the microphones have the same frequency characteristics. However, in practice, the frequency characteristics of microphones tend to vary from each other due to process variations in their production. For example, typical electret microphones can have variations of as much as 3 dB in the telephony frequency range. The performance of a CTDMA system degrades greatly if variations among the microphones exceed a range of 0.5-1.0 dB. Thus, extra measures are needed to calibrate such variations. While technically suitable calibration systems and methods are known, they tend to be costly in terms of hardware and time needed for operation, both of which are unacceptable for use in manufacture and test of low cost consumer electronics, such as cellular telephone handsets. Additionally, existing solutions are typically implemented with one or more analog-to-digital converters (ADCs) which couple the microphones to power consuming digital signal processor (DSP) systems performing powerful signal processing algorithms that, in turn, unavoidably degrade battery operating times.